A common form of density measuring instrument is the vibrating tube type wherein a hollow oscillator is filled with a sample under test. The density is determined from a parameter of the oscillator, typically the frequency or period of oscillation.
FIG. 1 illustrates a typical such prior art arrangement. A cantilevered hollow oscillator 1 is formed of transparent material and mounted to a support 2. This support forms a node in the oscillator which defines the volume of the sample under test. Means to sustain and measure the oscillation are well known and are omitted for clarity. Rectangular area 3 represents the area viewable by the operator through a window or viewport.
Sample entrance and exit ports 4 allow introduction of the sample under test. The sample flows around the U-tube shape as shown.
The presence of small bubbles or particulates anywhere in the oscillating sample can cause errors in the density measurement. Consequently, these instruments are often constructed with clear glass oscillators and viewing windows to allow the user to visually scan the sample for bubbles or particulates during and after injection of the sample under test.
The small physical size of the bubbles makes this difficult. Furthermore the density measuring apparatus is often placed in an environment that makes viewing inconvenient, for example under a fume hood.